Pocket no Naka no Oukoku

Pocket no Naka no Oukoku (ポケットの中の王国, Kingdom in Your Pocket) is a hybrid adventure/simulation game by Hect, best known as the developers of Moon Crystal. It's also possibly the most obscure commercially developed Game Boy game in existence.

The leaked game identifies itself internally as version 0.950, built on March 7th, 2001, and I would estimate that it's 75-80% done; it can be played from start to finish, but the last few areas are obviously incomplete and it has several glaring bugs.